Body size profoundly affects many aspects of animal biology, yet it remains one of the fundamental unsolved problems of developmental biology. Holometabolous insects - the primary model for the study of size regulation in animals - do not grow as adults, so the size at which larvae initiate metamorphosis determines their adult size. In holometabolous insect larvae, the decision to stop growing and metamorphose is attained at a particular weight, called the critical weight. Attainment of critical weight initiates a hormonal cascade that ultimately results in the synthesis and release of ecdysone, the hormone that coordinates the developmental events necessary for a larva to molt and metamorphose. The phenomenon of the critical weight has been observed for decades, and more recent research has elucidated the signaling pathways that regulate the synthesis of ecdysteroids. However, the mechanisms that a larva uses to sense its size and activate these signaling pathways are largely unknown. The result is a conspicuous gap in our understanding of the mechanisms that regulate body size. I hypothesize that, as larvae grow through an instar, the growth of tissues relative to supply structures creates internal hypoxia, and internal hypoxia is a physiological cue that initiates the hormonal cascade for molting and metamorphosis. Further, I hypothesize that oxygen effects on critical weight are mediated by hypoxia's interaction with the known pathways (insulin/IGF, TOR and Ras/Raf/MAPK) that regulate ecdysone secretion. I will investigate this question using physiological, morphological and molecular-genetic methods. First I will test whether the reduction in the critical weight in hypoxic conditions reflects a shift in the underlyig physiology of ecdysone signaling. I will test whether larvae become hypoxic later in the instar by measuring HIF-1 (hypoxia inducible factor) proteins using western blotting on whole flies, and by assessing HIF-1 signaling in different cell types using flies with GFP-reporter constructs. I will explore the role of HIF-1 signaling in regulating critical weight in normoxic and hypoxic conditions, using flies in which HIF-1 signaling is constitutively active or absent. Together, these experiments will test whether internal hypoxia is a critical cue in size-sensing and metamorphosis initiation in Drosophila. This study has the potential to explain phenomena such as why many species, including humans, have smaller body sizes at high altitude (low oxygen conditions). By elucidating the role of hypoxia in the normal regulation of growth and size, this study will also shed light on the mechanisms by which hypoxia regulates the pathological growth and size of cancer tumors.